636 RESEARCH PAPER
NOVEMBER, 636–638
JOURNAL OF CHEMICAL RESEARCH 2007
Spectroscopic and theoretical studies on symmetric aryl azines
Balakrishnan Karthikeyan*, Jayaraman Jayabharathi and Venugopal Thanikachalam
Department of Chemistry, Annamalai University, Annamalainagar 608 002, Taminadu, India
Azines, which closely resemble azobenzenes, have important applications in the field of nonlinear optics. NMR
and IR spectroscopic study of some symmetric aryl azines has been undertaken with the aid of ab initio theoretical
calculations. All data indicated that the structure of azine and substituted azines is the symmetric trans form.
Keywords: azines, vibrational spectra, ab initio calculations
using the MOLVIB program.14 The normal modes were analysed by
Azines are important in C–C bond formation and react as the
ene component in [3 + 2] additions.1 Azines with donor and
transforming the calculated displacements from Cartesian to internal
coordinate basis using a program, NMODES.15 The assignments of
acceptor groups [D–C6H4–(R)C=N–N=C(R)–C6H4–A] at the
the calculated frequencies were clarified by visual inspection of the
ends of a p-conjugated backbone behave as a novel non-linear
normal modes using the program MOLVIB. The vibrational
frequencies obtained are reported without scaling.
optical materials.2 Hagen et al.3 from the electron diffraction
analysis of the molecular structure of 2,3-diazabuta-1,3-diene
(CH2 = N-N=CH2) have shown that this compound exists as an
Results
equilibrium mixture of anti and gauche conformers whereas
IR, 1H and 13C NMR spectra were recorded for the azine whose
the corresponding tetrabromo derivative exists exclusively
structure is 1a. The signals in 1H and 13C spectra were assigned on their
in the gauche conformer. There are reports of solid state
positions, integrals, multiplicities and on comparison with those of the
corresponding parent aldehyde. There are two possible conformations
structures of several azines by X-ray diffraction studies4-6 and
configurational and conformational aspects of heterocyclic
azines by NMR techniques.7-9 Analysis of vibrational
spectroscopic signals along with NMR data will give
detailed information about the structures exhibited by azines.
of C=N groups, namely the s-trans and s-cis conformations, 2a and
2b. Several studies have revealed16-18 that the s-cis (2a) form is
destabilised due to strong interactions of the vicinal electron lone
pairs on the nitrogen atoms along with electrostatic effects and steric
repulsion of the two ends of groups attached to the sp hybridised
carbon atoms. Single crystal measurements reported by Mom and
Dewith6 have also revealed that the N(1),N(2)-bis(benzylidene)
azine exists in the s-trans conformation only.
In the present study the s-cis conformation is also ruled out. In the
s-trans conformation (2a) there are two possible configurations of the
aryl ring as shown in 3. In the isomer 3a both the azomethine protons
are syn to the N–N bond whereas in isomer 3b one hydrogen is syn to
the N–N bond and the other hydrogen is anti to the N–N bond. In the
isomer 3b, two different chemical shifts are expected for azomethine
protons whereas in the isomer 3a one signal is expected for a CH=N
1
In continuation of our earlier work10,11 on the H and 13C
NMR studies of azines, we now report on the spectroscopic
studies of symmetrical azines derived from substituted aryl
aldehydes.
Experimental
Materials and methods
The azines were prepared according to the standard procedure reported
in the literature.12 About 0.02 mol of the aryl aldehyde was refluxed
with 0.01 mol of hydrazine hydrate in 20 ml ethanol for 3 h and the
reaction mixture was poured into crushed ice; the precipitate obtained
was crystallised from an ethanol–chloroform (1:10) mixture. Yields
were in the range 70–85%. IR spectra were recorded on a Nicolet
Avatar 360 FT-IR spectrometer. The sample was mixed with KBr and
the pellet technique was adopted to record the spectra. Proton spectra
were recorded on a Bruker WH 400 NMR spectrometer at 400 MHz.
Samples were prepared by dissolving about 10 mg of material in
0.5 ml of CDCl3 containing 1% TMS. The experimental parameters
were the following: data points 32 K, number of transients 10; spectral
width 4000 Hz. Proton-decoupled 13C spectra were recorded on
Bruker WH 400 NMR spectrometer operating at 100 MHz using
10 mm sample tubes. Solutions for the measurement of spectra
were prepared by dissolving 0.5 g of the sample in 2.5 ml CDCl3
containing few drops of TMS as internal reference. The solvent
CDCl3 also provided the internal field frequency lock signal. The
experimental parameters were the following: number of scans 5000;
number of data points 32 K, spectral sweep width 22000 Hz, and
pulse width 6 ms.
1
proton. The observation of only one signal in the H NMR spectra
of the azine confirms that the azine exist in the isomeric form ‘a’
only. This form is further supported by the following observations.
Generally in azines protons which are syn to a N–N bond are expected
to resonate upfield compared to anti protons. Arnal et al.8 have
reported that acetal azine [CH3CH=N–N=CH–CH3] exists in only
one isomeric form in which both the azomethine protons are syn to
the N–N bond. The azomethine protons resonate upfield (7.89 ppm)
relative to the aldehydic proton in acetaldehyde (9.80 ppm). It is seen
from Table 1 that azomethine protons resonate considerably upfield
in all azines relative to their corresponding aldehydes. The magnitude
of shielding ranges from –1.11 to –1.44 ppm. This observation also
supports the symmetrical azine existing in the conformer 3a in which
both the azomethine protons are syn to the N–N bond.
Study of vibrational frequencies will illuminate the individual
vibrational modes which will help in the establishment of the
structure exhibited by azines and so ab initio vibrational analysis
has been carried out on the parent azine 1a. The azine has 28 atoms
having 78 vibrations. The optimised structure of 1a is shown as 4.
The theoretical IR intensity and Raman activity are plotted against
the calculated vibrational frequencies in Fig. 1.
The experimental IR spectra exhibited strong absorptions around
2920 (nC-H), 1620 (nC=N) and 1210 (nC=N-N)cm-1. From the visual
assignments from the Molvib programme it is inferred that the
higher frequency range vibrations (above 3000 cm-1) are due to ring
CH symmetric stretching while the frequencies in the range below
1700cm-1 areduetomixedmodes.The1649and1659cm-1 absorptions
of the calculated frequencies are assigned to the symmetric C=N stretch
and asymmetric C=N vibration. As these frequencies are mutually
present in both the IR and Raman spectra we can confirm that the
azine is in the C2h symmetry. The theoretical vibrational frequencies
327, 550, 1230, 1530 cm-1 were mainly due to the N–N atom pair.
It is expected19 that the symmetric N–N stretching vibration will be at
1200–1000 cm-1 as a strong Raman line and the absence of this line in
the experimental IR spectra further confirms the above conclusion.
Theoretical calculations
The theoretical calculations presented here were performed with the
Gaussian-94/DFT 13program on an IBM-RS6000 computer system.
The molecular geometry of the parent azine was optimised using
the HF method with the basis set 6-31G*. A complete geometry
optimisation was carried out employing Berny’s optimisation
algorithm, which resulted in CS symmetry. The vibrational
frequencies and corresponding normal modes were then evaluated
at the optimised geometry using analytical differentiation algorithms
contained within the program. The assignment of the calculated
normal modes was made from the corresponding potential energy
distributions (PEDs) and isotopic shifts. The PEDs and frequencies
of the isotopically labelled species were calculated from the
quantum mechanically derived Cartesian force constant matrix
* Correspondent. E-mail: bkarthi_au@yahoo.com
PAPER: 07/4852
Karthikeyan, Balakrishnan
